As a result of development of a silica optical fiber of low loss, many optical fiber communication systems wherein an optical fiber is used for a transmission line have been put into practical use. An optical fiber itself has a very broad band. However, the transmission capacity by an optical fiber is actually limited by system designing. The most significant limitation arises from waveform distortion by chromatic dispersion which occurs in an optical fiber. Further, while an optical fiber attenuates an optical signal, for example, at the rate of approximately 0.2 dB/km, the loss by such attenuation has been compensated for by adoption of optical amplifiers including an erbium-doped fiber amplifier (EDFA).
Chromatic dispersion often simply called dispersion is a phenomenon wherein the group velocity of an optical signal in an optical fiber varies as a function of the wavelength (frequency) of the optical signal. For example, in a standard single mode fiber, where the wavelength is shorter than 1.3 .mu.m, an optical signal having a longer wavelength propagates faster than another optical signal having a shorter wavelength, and dispersion as a result of this is usually called normal dispersion. Where the wavelength is longer than 1.3 .mu.m, an optical signal having a shorter wavelength propagates faster than another optical signal having a longer wavelength, and dispersion as a result of this is called anomalous dispersion.
In recent years, originating from an increase in optical signal power by adoption of an EDFA, attention is paid to the nonlinearity. The most significant nonlinearity of an optical fiber which limits the transmission capacity is an optical Kerr effect. The optical Kerr effect is a phenomenon wherein the refractive index of an optical fiber varies in accordance with the intensity of an optical signal. The variation of the refractive index modulates the phase of an optical signal which propagates in an optical fiber, and as a result, frequency chirping which varies the signal spectrum occurs. This phenomenon is known as self-phase modulation (SPM). The spectrum is expanded by the SPM, by which the waveform distortion by chromatic dispersion is further increased.
In this manner, the chromatic dispersion and the Kerr effect provide waveform distortion to an optical signal as the transmission distance increases. Accordingly, in order to allow long-haul transmission by an optical fiber, it is required that the chromatic dispersion and the nonlinearity be controlled, compensated for or suppressed.
As a technique for controlling the chromatic dispersion and the nonlinearity, a technique which employs a regenerative repeater which includes an electronic circuit for a main signal is known. For example, a plurality of regenerative repeaters are disposed intermediately of a transmission line, and in each of the regenerative repeaters, photo-electric conversion, regeneration processing and electro-optical conversion are performed in this order before the waveform distortion of the optical signal becomes excessive. This method, however, has a problem in that a regenerative repeater which is expensive and complicated is required and an electronic circuit of the regenerative repeater limits the bit rate of a main signal.
As a technique for compensating for the chromatic dispersion and the nonlinearity, a light soliton is known. Optical signal pulses having an amplitude, a pulse width and a peak power defined accurately with respect to a given value of the anomalous dispersion are generated, and consequently, a light soliton propagates while it maintains its waveform because of balancing between pulse compression which arises from the SPM and the anomalous dispersion by the optical Kerr effect and pulse expansion by the dispersion.
As another technique for compensating for the chromatic dispersion and the nonlinearity, application of optical phase conjugation is available. For example, a method for compensating for the chromatic dispersion of a transmission line has been proposed by Yariv et al. (A. Yariv, D. Fekete, and D. M. Pepper, Compensation for channel dispersion by nonlinear optical phase conjugation" Opt. Lett., vol. 4, pp. 52-54, 1979). An optical signal is converted into phase conjugate light at a middle point of a transmission line, and waveform distortion by chromatic dispersion which the optical signal has undergone in the former half of the transmission line is compensated for by distortion by chromatic dispersion in the latter half of the transmission line.
Particularly, if it is assumed that the factors of the phase variation of an electric field at two locations are same and the variation in environment which brings about the factors is moderate within a transmission time of light between the two locations, then the phase variation is compensated for by disposing a phase conjugator (phase conjugate light generating apparatus) intermediately between the two locations (S. Watanabe, "Compensation of phase fluctuation in a transmission line by optical conjugation" Opt. Lett., vol. 17, pp. 1,355-1,357, 1992). Accordingly, by adoption of a phase conjugator, also waveform distortion which arises from SPM is compensated for. However, where the distribution of the optical power is asymmetrical before and after the phase conjugator, the compensation for the nonlinearity becomes incomplete.
The inventor of the present invention has proposed a technique for overcoming the incompleteness of the compensation by the nonlinearity of the optical power where a phase conjugator is used (S. Watanabe and M. Shirasaki, "Exact compensation for both chromatic dispersion and Kerr effect in a transmission fiber using optical phase conjugation" J. Lightwave Technol., vol. 14, pp. 243-248, 1996). A phase conjugator is disposed in the proximity of a point of a transmission line before and after which the total amounts of the dispersion values or the nonlinear effect are equal, and various parameters before and after the point are set for each small interval. However, since a phase conjugator is disposed intermediately of the transmission line, where the transmission line is laid between continents, for example, the phase conjugator may possibly be laid on the bottom of the sea. In this instance, maintenance of the phase conjugator is difficult. It may be proposed to dispose a front half portion or a rear half portion of a transmission line in a transmission terminal station or a reception terminal station and lay the remaining half of the transmission line between continents. In this instance, since the phase conjugator can be provided in the transmission terminal station or the reception terminal station, maintenance of it is easy. However, in this instance, a deviation may appear in setting of parameters between the front half portion and the rear half portion of the transmission line and may make the compensation incomplete.
It is an object of the present invention to provide an optical fiber communication system wherein the chromatic dispersion and the nonlinearity can be compensated for effectively by using two or more phase conjugators.
It is another object of the present invention to provide an optical fiber communication system wherein a phase conjugator need not be disposed intermediately of a transmission line in order to compensate for the chromatic dispersion and the nonlinearity.
Other objects of the present invention become apparent from the following description.